The present invention relates to a method for producing an aldehyde or a ketone by the Oppenauer oxidation from a corresponding allyl alcohol as a starting material.
Processes of oxidation of allyl alcohols to give corresponding aldehydes or ketones are important reactions in the field of organic synthesis. A typical example of the oxidations includes the Oppenauer oxidation. The Oppenauer oxidation comprises a reaction in which a carbonyl compound as a hydride acceptor and an alcohol to be oxidized are converted into an alcohol and a carbonyl compound via a 6-membered transition state with hydride transfer in the presence of a basic catalyst such as metal alkoxides. Acetone, cyclohexanone, benzaldehyde and the like are known as the hydride acceptor, and metal alkoxides such as aluminum alkoxides and potassium tert-butoxide are known as the catalyst.
This reaction is highly selective and free from influence on a carbon-carbon double and triple bonds, amino groups, halogen, cyano groups, acetal groups, acyl groups and the like. Therefore, the reaction is an extremely useful compared to many other oxidations. However, this reaction suffers from some drawbacks. For example, this reaction is performed at a high temperature, and it requires a large excess of a hydride acceptor and a more than stoichiometric amount of a catalyst.
In order to solve these problems, reactions have recently been reported in which a catalytic amount of zirconium alkoxide (K. Krohn, et al., Synthesis 1996, 1341), zirconium complex (Y. Ishii, et al., J. Org. Chem. 1986, 51, 240), ruthenium complex (M. L. S. Almeida, et al., J. Org. Chem. 1996, 61, 6587), samarium alkoxide (J. L. Namy, et al., J. Org. Chem. 1984, 49, 2045), arylboron compounds (Japanese Patent Unexamined Publication (Kokai) No. 11-228479) or aluminum compounds (T. Ooi, et al., Angew. Chem. Int. Ed. 1998, 37, 2347) is used as the Oppenauer oxidation catalyst. However, industrial applications of these catalysts are limited from a viewpoint of their costs.
Another reaction has been reported in which aluminum alkoxide, a rather inexpensive material, is used in a catalytic amount and furfural is used as a hydride acceptor (Japanese Patent Unexamined Publication No. 51-141801). However, in the aforementioned method, an alcohol as a starting material cannot be completely converted into a corresponding carbonyl compound, which arises a problem from a viewpoint of a reaction yield.
A process is also reported in which rather inexpensive aluminum alkoxide is used in a catalytic amount and a tert-aldehyde is used as a hydride acceptor (U.S. Pat. No. 4,663,488). However, industrial application of said method is limited from a viewpoint of a price of the hydride acceptor.
Furthermore, a method is reported in which diisopropoxyaluminum trifluoroacetate, prepared by adding trifluoroacetic acid to aluminum isopropoxide, is used as a catalyst and p-nitrobenzaldehyde is used as a hydride acceptor (K. G. Akamanchi, et al., Tetrahedron Lett. 1997, 38, 6925). However, this method utilizes a stoichiometric amount of the catalyst, and oxidation of allyl alcohols is neither taught nor suggested in the report.
An object of the present invention is to solve the aforementioned problems and to provide a method for oxidation of allyl alcohols to corresponding aldehydes or ketones which is industrially advantageous.
Under the circumstances, the inventors of the present invention conducted various researches to find an industrially advantageous method for oxidation of allyl alcohols to corresponding aldehydes or ketones, i.e., a method utilizing an inexpensive aluminum alkoxide and a hydride acceptor. As a result, they found a novel method for oxidation of allyl alcohols and thus achieved the present invention.
The present invention thus provides a method for producing an aldehyde or a ketone represented by the following general formula [I]: 
wherein R1 represent hydrogen atom, an alkyl group having 1 to 6 carbon atoms or an alkenyl group having 2 to 11 carbon atoms, R2 represent an alkyl group having 1 to 6 carbon atoms, an alkenyl group having 2 to 16 carbon atoms, or an aryl group having 6 to 10 carbon atoms. R3 and R4 independently represent hydrogen atom, and wherein R1 and R4 may bind to each other to form an alkylene ring having 1 to 5 carbon atoms and/or R2 and R3 may bind to each other to form an alkylene ring having 1 to 6 carbon atoms, which comprises a step of reacting an allyl alcohol represented by the following general formula [II]: 
wherein R1, R2, R3 and R4 have the same meanings as those defined above with a hydride acceptor represented by the following general formula [III]: 
wherein X and Y independently represent hydrogen atom, a halogen atom, or nitro group, provided that said hydride acceptor wherein both of X and Y are simultaneously represent hydrogen atoms is excluded, in the presence of an aluminium alkoxide as an Oppenauer oxidation catalyst.
According to preferred embodiments of the present invention, provided are:
the aforementioned method wherein, in the allyl alcohol represented by the general formula [II], R1 represents hydrogen atom or methyl group, R2 represents phenyl group or a group represented by the following general formula [IV]: 
wherein m represents an integer of from 1 to 3 and - - - - represents a single bond or a double bond, and each of R3 and R4 represents hydrogen atom, or wherein, in the allyl alcohol represented by the general formula [II], R1 represents a group represented by the general formula [IV], R2 represents hydrogen atom or methyl group, and each of R3 and R4 represents hydrogen atom;
the aforementioned method, wherein the allyl alcohol represented by the general formula [II] is (2E,6E)-3,7,11-trimethyl-2,6,10-dodecatrien-1-ol;
the aforementioned method, wherein the hydride acceptor represented by the general formula [III] is a hydride acceptor selected from 3-nitrobenzaldehyde, 2-nitrobenzaldehyde, 2-fluorobenzaldehyde and 2-bromobenzaldehyde;
the aforementioned method, wherein the hydride acceptor represented by the general formula [III] is 2-nitrobenzaldehyde
the aforementioned method, wherein the Oppenauer oxidation catalyst is selected from aluminum isopropoxide, aluminum tert-butoxide, aluminum phenoxide, and aluminum sec-butoxide; and
the aforementioned method, wherein the Oppenauer oxidation catalyst is aluminum isopropoxide.
By the method of the present invention, allyl alcohols can be converted to corresponding aldehydes or ketones in a high yield under a mild condition using an inexpensive catalyst and a hydride acceptor. Therefore, the aforementioned method of the present invention has industrial advantages.
In the aforementioned formulas, R1, R2, R3 and R4 represent hydrogen atom or a hydrocarbonic group. Examples of the hydrocarbonic group include, for example, an alkyl group, an alkenyl group, an alkynyl group, a cycloalkyl group, a cycloalkenyl group, an aryl group and the like. Preferred alkyl groups are those having 1 to 6 carbon atoms, preferred alkenyl groups are those having 2 to 16 carbon atoms, and they may be linear or branched. Preferred aryl groups are those having 6 to 10 carbon atoms.
Examples of the alkyl group include, but not limited to, methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, n-pentyl, neopentyl, 2-methylbutyl, 1-methylbutyl, 1-ethylpropyl, 1,1-dimethylpropyl, n-hexyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 3,3-dimethylbutyl, 2,2-dimethylbutyl, 1,1-dimethylbutyl, 2-ethylbutyl, 1-ethylbutyl, 1,3-dimethylbutyl and the like.
Examples of the alkenyl group include, but not limited to, vinyl, 1-propenyl, 2-propenyl, isopropenyl, 1-butenyl, 2-butenyl, 2-methyl-2-butenyl, 1-pentenyl, 1-hexenyl, hexadienyl, heptenyl and the like.
Examples of the alkynyl group include, but not limited to, ethynyl, propynyl, butynyl, hexynyl, decynyl and the like. Examples of the cycloalkyl group include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl and the like. Examples of the cycloalkenyl group include, for example, cyclopropenyl, cyclobutenyl, cyclopentenyl, cyclohexenyl, cyclohexadienyl, cycloheptenyl, cyclodecenyl and the like.
Examples of the aryl group include, but not limited to, phenyl, naphthyl and the like.
The aforementioned alkyl group, alkenyl group and alkynyl group may be substituted with 1 to 4 functional groups selected from, for example, the aforementioned cycloalkyl group, cycloalkenyl group, aryl group and the like. The aforementioned cycloalkyl group and cycloalkenyl group may be substituted with 1 to 4 functional groups selected from, for example, the aforementioned alkyl group, alkenyl group, alkynyl group, and aryl group. The aforementioned aryl group may be substituted with 1 to 4 functional groups selected from the aforementioned alkyl group, alkenyl group, alkynyl group, cycloalkyl group, cycloalkenyl group and the like.
In the present invention, R1 and R2 may bind to each other to form a cyclic hydrocarbon. Examples of the cyclic hydrocarbon formed include, but not limited to, saturated cyclic hydrocarbons such as cyclopropane, cyclobutane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, cyclononane and cyclodecane, unsaturated cyclic hydrocarbons such as cyclopropene, cyclobutene, cyclopentene, cyclohexene, cyclohexadiene, cycloheptene and cyclodecene and the like. The aforementioned rings may be substituted with 1 to 4 functional groups selected from, for example, the aforementioned alkyl group, alkenyl group, alkynyl group, aryl group and the like.
R1 and R4 may bind to each other to form an unsaturated cyclic hydrocarbon, and/or R2 and R3 may bind to each other to form an unsaturated cyclic hydrocarbon. Examples of the unsaturated cyclic hydrocarbon formed include, but not limited to, cyclopropene, cyclobutene, cyclopentene, cyclohexene, cyclohexadiene, cycloheptene, cyclooctene, cyclooctadiene, cyclodecene, norpinene, norbornene and the like. Each of the aforementioned unsaturated cyclic hydrocarbons may be substituted with 1 to 4 functional groups selected from, for example, the aforementioned alkyl group, alkenyl group, alkynyl group, aryl group and the like.
Specific examples of the group represented by the general formula [IV] include, but not limited to, 4-methylpenta-1,3-dienyl, 4-methylpenta-1-enyl, 4-methylpenta-3-enyl, 4-methylpentyl, 4,8-dimethylnona-1,3,5,7-tetraenyl, 4,8-dimethylnona-1,3,7-trienyl, 4,8-dimethylnona-3,5,7-trienyl, 4,8-dimethylnona-3,7-dienyl, 4,8-dimethylnona-7-enyl, 4,8-dimethylnona-3-enyl, 4,8-dimethylnonyl, 4,8,12-trimethyltrideca-1,3,5,7,9,11-hexenyl, 4,8,12-trimethyltrideca-1,3,5,7,11-pentenyl, 4,8,12-trimethyltrideca-1,3,7,9,11-pentenyl, 4,8,12-trimethyltrideca-3,5,7,9,11-pentenyl, 4,8,12-trimethyltrideca-1,3,7,11-tetraenyl, 4,8,12-trimethyltrideca-3,5,7,11-tetraenyl, 4,8,12-trimethyltrideca-3,7,9,11-tetraenyl, 4,8,12-trimethyltrideca-5,7,9,11-tetraenyl, 4,8,12-trimethyltrideca-1,3,9,11-tetraenyl, 4,8,12-trimethyltrideca-1,3,5,7-tetraenyl, 4,8,12-trimethyltrideca-3,9,11-trienyl, 4,8,12-trimethyltrideca-3,5,7-trienyl, 4,8,12-trimethyltrideca-7,9,11-trienyl, 4,8,12-trimethyltrideca-1,3,7-trienyl, 4,8,12-trimethyltrideca-1,3,11-trienyl, 4,8,12-trimethyltrideca-5,7,11-trienyl, 4,8,12-trimethyltrideca-9,11-dienyl, 4,8,12-trimethyltrideca-5,7-dienyl, 4,8,12-trimethyltrideca-1,3-dienyl, 4,8,12-trimethyltrideca-3,11-dienyl, 4,8,12-trimethyltrideca-7,11-dienyl and 4,8,12-trimethyltrideca-3,7-dienyl.
R1 may preferably be hydrogen atom, the aforementioned alkyl group having 1 to 6 carbon atoms, the aforementioned alkenyl group having 2 to 11 carbon atoms, the aforementioned aryl group having 6 to 10 carbon atoms, or a group represented by the general formula [IV] wherein symbol xe2x80x9cmxe2x80x9d is preferably 1 or 2, more preferably 1, and number of double bonds is preferably 1 to 3, more preferably 1.
R1 is more preferably hydrogen atom, methyl or 4-methylpenta-3-enyl.
R2 may preferably be the aforementioned alkyl group having 1 to 6 carbon atoms, the aforementioned alkenyl group having 2 to 16 carbon atoms, the aforementioned aryl group having 6 to 10 carbon atoms, or a group represented by the general formula [IV] wherein symbol xe2x80x9cmxe2x80x9d is preferably 1 to 3, more preferably 1 or 2, and number of double bonds is preferably 1 to 4, more preferably 1 or 2.
R2 is more preferably methyl, 4,8-dimethylnona-3,7-dienyl, 4-methylpenta-3-enyl or phenyl.
R3 and R4 may preferably be hydrogen atom.
In the present invention, it is preferred that R1 and R4 or R2 and R3 bind to each other to form the aforementioned unsaturated cyclic hydrocarbon having 3 to 8 carbon atoms, preferably 3 to 6 carbon atoms, which may be substituted. Most preferably R1 and R4 bind to each other to form dimethylbicycloheptene or R2 and R3 bind to each other to form isopropenylcyclohexene.
In the present invention, the reaction can be performed without a solvent or in a solvent. Examples of the solvent include, for example, an ether such as diethyl ether, diisopropyl ether, tetrahydrofuran, dioxane, dimethoxyethane, 2-methoxyethyl ether and petroleum ether, aromatic hydrocarbon such as benzene and toluene, halogenated hydrocarbon such as dichloromethane and chloroform, aliphatic hydrocarbon such as hexane, heptane and octane, cycloaliphatic hydrocarbon such as cyclohexane or a mixed solvent thereof. An aromatic hydrocarbon such as benzene or a cycloaliphatic hydrocarbon such as cyclohexane may preferably be used. The reaction is preferably performed at a reaction temperature of from xe2x88x9220xc2x0 C. to a temperature around a boiling point of a solvent used, preferably in a range of from 5 to 30xc2x0 C. Reaction time may usually be 1 to 24 hours, which may vary depending on reaction conditions.
In the present invention, an aluminum alkoxide is used as the Oppenauer oxidation catalyst. Preferred examples of the aluminium alkoxide include aluminum isopropoxide, aluminum tert-butoxide, aluminum phenoxide, and aluminum sec-butoxide, and a more preferred example includes aluminum isopropoxide. The Oppenauer oxidation catalyst is used in an amount less than a stoichiometric amount, preferably in an amount of from 0.01 to 0.90 molar equivalent, more preferably in an amount of from 0.05 to 0.50 molar equivalent, most preferably in an amount of 0.1 molar equivalent.
In the present invention, the hydride acceptor represented by the aforementioned general formula [III] is used. In the aforementioned general formula [III], X and Y independently represent hydrogen atom, a halogen atom, or nitro group. Examples of the halogen atom include fluorine, chlorine, bromine, and iodine atoms, and fluorine and bromine are preferred. When one of X and Y represents hydrogen atom, the halogen atom or nitro group preferably substitutes at the 2- or 3-position.
Examples of the hydride acceptor represented by the general formula [III] include 2-nitrobenzaldehyde, 3-nitrobenzaldehyde, 2-bromobenzaldehyde, 3-bromobenzaldehyde, 4-bromobenzaldehyde, 2-chlorobenzaldehyde, 3-chlorobenzaldehyde, 4-chlorobenzaldehyde, 2-fluorobenzaldehyde, 3-fluorobenzaldehyde, 4-fluorobenzaldehyde, 2,3-dibromobenzaldehyde, 2,4-dibromobenzaldehyde, 2,5-dibromobenzaldehyde, 2,6-dibromobenzaldehyde, 2,3-dichlorobenzaldehyde, 2,4-dichlorobenzaldehyde, 2,5-dichlorobenzaldehyde, 2,6-dichlorobenzaldehyde, 2,3-difluorobenzaldehyde, 2,4-difluorobenzaldehyde, 2,5-difluorobenzaldehyde and 2,6-difluorobenzaldehyde. Preferred examples include 3-nitrobenzaldehyde, 2-nitrobenzaldehyde, 2-fluorobenzaldehyde and 2-bromobenzaldehyde, and a most preferred example includes 2-nitrobenzaldehyde. The hydride acceptor is used in an amount not less than a stoichiometric amount, preferably in an amount of 1 to 5 molar equivalents, more preferably in an amount of 1.1-1.5 molar equivalents.
A reaction product can be isolated and purified by a suitable combination of usual means including centrifugation, concentration, phase separation, washing, desiccation, recrystallization, distillation, column chromatography and the like.
The target compound represented by the general formula [I] obtained by the aforementioned method of the present invention is useful as a starting material for preparation of various compounds. In particular, the target compound of the general formula [I] having a group represented by the general formula [IV] may be used as a starting material for preparation of polyisoprenoid derivatives which are useful as anticancer agents and the like.
(2E,4E,6E,10E)-3,7,11,15-Tetramethyl-2,4,6,10,14-hexadecapentaenoic acid (NIK-333), which is one of polyisoprenoid derivatives, is known to have transcription activation activities through retinoid receptors, and have differentiation inducing and apoptosis inducing activity on hepatocellular carcinoma. Clinically, long term administration of NIK-333 for 1 year significantly suppressed recurrence of hepatic cancer after curative treatment, suggesting its suppressing activity on hepatic cancer recurrence. Moreover, liver function failure and side effect such as those in therapies with other retinoids were not significantly observed in the clinical application (N. Eng. J. Med., 334, 1561-1567 (1996)).
The method of the present invention is extremely useful, for example, from a standpoint that the method enables manufacture of (2E,6E)-3,7,11-trimethyl-2,6,10-dodecatrien-1-al, which is an expensive synthetic intermediate for NIK-333, in a high yield from inexpensive (2E,6E)-3,7,11-trimethyl-2,6,10-dodecatrien-1-ol.